Memory is widely used in various electronic devices such as cellular telephones, digital cameras, personal digital assistants, medical electronics, mobile computing devices, non-mobile computing devices and data servers. Memory may comprise non-volatile memory or volatile memory. A non-volatile memory allows information to be stored and retained even when the non-volatile memory is not connected to a source of power (e.g., a battery).
One example of a non-volatile memory is magnetoresistive random access memory (MRAM), which uses magnetization to represent stored data, in contrast to other memory technologies that use electronic charges to store data. Generally, MRAM includes a large number of magnetic memory cells formed on a semiconductor substrate, where each memory cell represents one data bit. A bit of data is written to a memory cell by changing the direction of magnetization of a magnetic element within the memory cell, and a bit is read by measuring the resistance of the memory cell (e.g., low resistance typically represents a “0” bit and high resistance typically represents a “1” bit).
FIG. 1A is a schematic perspective view of an example prior art MRAM memory cell 100 that makes use of spin orbit torque (SOT) for switching. In general, Spin Hall Effect (SHE) may be used to generate spin current flowing in a direction transverse to the direction of an applied electrical current. Memory cell 100 includes three terminals A, B and C; magnetic tunnel junction (MTJ) 101; and SHE layer 120. MTJ 101 comprises pinned layer (PL) 102, inter-layer coupling (ILC) layer 104, reference layer (RL) 106, tunnel barrier (TB) 108 and free layer (FL) 110. For an in-plane SOT memory cell, free layer (FL) 110 has a direction of magnetization that can be switched between INTO the page and OUT of the page. Reference layer (RL) 106 has a direction of magnetization that is INTO the page. Pinned layer (PL) 102 has a direction of magnetization that is OUT of the page. The ILC layer 104 promotes a strong antiferromagnetic (i.e., anti-parallel) coupling between PL (102) and RL (106), such that their net magnetic moment mostly cancels, thus greatly reducing unwanted stray field influence on the free layer.
To write data to memory cell 100, a write current 122 is applied between terminal B and terminal C. Reading is achieved by passing current between terminal A and terminal B in order to sense the resistance of memory cell 100. If the direction of magnetization of free layer (FL) 110 is parallel to the direction of magnetization of the RL (106), for example INTO the page, then memory cell 100 has a lower resistance. If the direction of magnetization of free layer (FL) 110 is antiparallel to the direction of magnetization of the RL (106), for example OUT the page, then memory cell 100 has a higher resistance.
FIG. 1B is a top view of memory cell 100, with bidirectional arrow 130 indicating the switchable direction of magnetization of free layer 110. As depicted, the shape of MTJ 101 is elliptical in order to maintain thermal stability. Namely, the elliptical shape of the FL introduces magnetic shape anisotropy which provides energy barrier against thermally activated magnetization reversal of the FL thus making FL magnetization thermally stable. The shape of SHE layer 120 is rectangular.
The primary advantage of the SOT-switching design that exploits the SHE is that the write current 122 passes solely through the SHE layer 120, and does not flow through the tunnel barrier 108. This avoids long term degradation of the tunnel barrier by the switching current. However, as the size of memory cell 100 is scaled down, the memory cell 100 loses its ability to retain data. This is because the magnetic shape anisotropy energy is directly proportional to the volume of the FL and as this volume is reduced the energy barrier against thermally activated magnetization reversal decreases, eventually to the point that thermally stable magnetization of the FL cannot be maintained.